From this question I learned that Google App Engine does not currently support SSL on \"custom domains\" (at least not as of June 2010, when that question was asked).
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All secure traffic with Google App Engine must be served from your appspot.com domain (https://your-app-id.appspot.com). If you are serving your app off of a Google Apps domain, you must direct all secure traffic through your app's appspot domain.
This is what is written in google app engine documentation. That means. SSL is supported on appspot.com domain